The process of fermentation production of chemical products using sugars as raw materials has been used to produce various industrial materials. At present, as the sugars to be used as fermentation feedstocks, those derived from food materials such as sugar cane, starch and sugar beet are industrially used. However, in view of the fact that rise in the prices of food materials is expected due to future increase in the world population, or in an ethical view of the fact that sugars for industrial materials may compete with sugars for food, a process of efficiently producing a sugar liquid from a renewable nonfood resource, that is, cellulose-containing biomass, or a process of using the obtained sugar liquid as a fermentation feedstock to efficiently convert it to an industrial material, needs to be constructed in the future.
As the prior art for obtaining sugar from biomass, methods wherein concentrated sulfuric acid is used to hydrolyze cellulose and hemicellulose contained in the biomass into monosaccharides represented by glucose and xylose (Japanese Translated PCT Patent Application Laid-open No. 11-506934 and JP 2005-229821 A), and methods wherein pretreatment is carried out for improving the reactivity of biomass, followed by hydrolysis of the biomass by enzymatic reaction (JP 2001-95594 A and JP 3041380 B) are generally known. In such cases, in hydrolysis of a cellulose-containing biomass, decomposition of the cellulose and hemicellulose components and the like proceeds while decomposition reaction of produced sugars such as glucose and xylose proceeds, leading to production of by-products such as furan compounds including furfural and hydroxymethylfurfural, and organic acids including formic acid and acetic acid, which is problematic. These compounds have inhibitory actions during the fermentation step using a microorganism and cause inhibition of the growth of the microorganism, leading to a decreased yield of the fermentation product. Therefore, these compounds are called fermentation inhibitors and have been seriously problematic when a sugar liquid derived from a cellulose-containing biomass was used as a fermentation feedstock. As a method of removing such fermentation inhibitors in the sugar liquid production process, a method by removing fermentation inhibitors with a nanofiltration membrane or reverse osmosis membrane is known (WO 2010/067785).